This plugin hasnt been tested with the latest 3 major releases of WordPress. It may no longer be maintained or supported and may have compatibility issues when used with more recent versions of WordPress.

WP Better Emails

Description

All emails from WordPress (lost password, notifications, etc.) are sent by default in text/plain format. WP Better
Emails wraps them with a much better looking customizable HTML email template and lets you also set your own sender name and email address.

WP Better Emails comes with a default simple and clean template that has been tested on various and popular email clients
like Gmail, Yahoo Mail, Hotmail/Live, AOL, Outlook, Apple Mail and many more. This to ensure your emails will always display
nicely in your recipient mailbox. But you can of course design your own.

The key of the array sponsored_link will be a new tag (%sponsored_link%) you can include. It will be randomly replaced with one of your sponsored links.

The example above is taking sponsored links as an additinonal content but you can imagine anything like including lastest posts, a quote of the day or whatever.
You can place this function in your functions.php theme file or in a plugin.

Do you provide hooks to alter some WP Better Emails behaviours?

wpbe_html_body (filter) will let you apply your own logic on HTML emails

wpbe_plaintext_body (filter) will let you apply your own logic on plain text emails

But missing one feature:
I want to set it up in a multisite once, for all blogs. Admins of (sub) blogs should not be able to edit the template. So I need a general multisite plugin interface, where I can decide wether I want to use this setting over all blogs or if every blog can set it up for themselves.

This plugin didn't worked at all with my setting. I'm using Mailgun Plugin, don't know if that's what is causing the issue.
Only send text message with not any of the templates set on the Plugin Settings... no matter how many times you save the template, it is totally ignored.